The present invention relates to methods of and apparatus for regulating electron-beam welding generators of the kind having three electrodes, viz: a cathode, a Wehnelt electrode and an anode.
In certain types of electron-beam welding, in particular when welding circular parts, to eliminate the defects caused by sudden rises and falls in power it is found necessary to bring about a gradual rise in power followed by a period in which power is substantially constant (or variable to suit the configuration of the part), and then a gradual drop in power at the end of the welding operation.
In the case of high speed welding operations, or in the case of the production of parts which are difficult to weld, it has proved necessary, in order to achieve the minimum number of rejects, to regulate the principal welding parameters, namely the high accelerating voltage, the welding current put out by the gun, and the current to the magnetic focussing lens to values which are as constant as possible.
This regulation of welding power is generally carried out by one or other of the following two methods: on the one hand, only the value of the high voltage may be varied, which results in a corresponding variation in the welding current and thus in the power put out by the electron gun. This is what is called operation in the "diode" mode,
On the other hand, only the electron welding current emitted by the gun may be varied while holding the high voltage constant, by biasing the Wehnelt tube to be negative with respect to the cathode to a greater or lesser degree. This is called operation in the "triode" mode.
Both of these methods have their respective advantages and disadvantages.
The principal advantage of operation in the "diode" mode lies in the fact that the point of electrostatic concentration or "crossover" point of the electron beam remains substantially unaltered during regulation, and if it is located where the passage through the anode is situated, this passage can be made as narrow as possible. This results in a considerable increase in the pressure difference between the inter-electrode space and the remainder of the gun, thus enabling a high vacuum of the order of 10.sup.-5 Torr to be maintained in the inter-electrode space, which is particularly sensitive by reason of the danger of arcing. The remainder of the electron gun, that is to say the zone where the electromagnetic lens is situated, and even the welding chamber, may be maintained at a lower vacuum, termed a primary vacuum, of the order of 10.sup.-3 to 10.sup.-2 Torr.
The principal disadvantage of operation in the "diode" mode is that power may vary substantially not only as a function of variations in the high voltage but also as a function of the perveance of the gun, the perveance being in particular a function of dimensional variations of thermal origin in certain parts of the electron gun and also of the aging of the cathode. To compensate for these variations in power it is therefore necessary to regulate the high voltage, which results in relatively complicated arrangements which are unsuitable for use where the operations of modulating the power take place at a high frequency, as is often required in electron-beam welding operations.
On the other hand, operation in the "triode" mode enables the easy regulation of the welding current simply by altering the bias of the Wehnelt electrode, thus giving a welding power which is regulated to a value which may be constant. However, the considerable variations in the bias of the Wehnelt electrode are accompanied by a shift in the point of electrostatic focus and an increase in the angular width of the electron beam. Accordingly, to prevent the anode from heating up excessively, it is necessary to increase its conductance substantially, or in other words to increase the diameter of the passage through it, which has a particularly adverse effect on maintaining a high vacuum in the inter-electrode space.
It is an object of the invention to alleviate these disadvantages by using a suitable combination of both these methods of regulation which in all cases allows mainly their advantages to come into play without the disadvantages which have been mentioned being thereby incurred.